Conventionally, as an electron lens for use in, e.g., a SEM and an ion gun, there are known a magnetic lens and an electrostatic lens as disclosed in Patent Literatures 1 and 2. In the former magnetic lens, although an aberration can be reduced, it is very difficult to reduce a size and weight due to a limitation in shape of a magnetic pole. Meanwhile, although the latter electrostatic lens holds superiority in reducing the size and weight, it is difficult to reduce an aberration in a deceleration type and it is necessary to apply a high voltage to an electrode in an acceleration type, and therefore a pressure-resistant design thereof is not easy.
For example, in the case of a unipotential electrostatic lens of a conventional three-electrode deceleration type, if a lens design is performed as to have a minimum spherical aberration coefficient CS, chromatic aberration coefficient CC and the like with respect to a focal length F, CS=7.1×F is obtained. And assuming that an electron accelerating voltage VA=5 kV, a focal length F=5 mm, a radius r=4.54 mm of each through aperture in FIG. 9 and an inter-electrode distance s=an intermediate electrode thickness t=2.27 mm, CS=35.54 mm is obtained with a lens electrode voltage −8250V.
Compared this value (CS=35.54 mm) to CS=4.3 mm of a magnetic type lens of a strong magnetization with a bore size of 10 mm, an inter-pole gap of 5 mm and an identical focal length, the spherical aberration coefficient of the deceleration type electrostatic lens becomes approximately 8.3 times the spherical aberration coefficient of the magnetic type lens.
The reason why the spherical aberration coefficient of the deceleration type electrostatic lens becomes large like this is because, when electrons incident in parallel with a central axis keeping a distance r0 therefrom sequentially pass through an incident side electrode V1, an intermediate electrode V2 and a projection side electrode V3, the electrons are affected by these electrodes (forces of electric fields) and advance in a direction apart from a central axis m in an interval between the incident side electrode V1 and the intermediate electrode V2, and after a separation distance r from the central axis m becomes maximum, the electrons converge while tracing an orbit like a mountain in a manner of proceeding toward or intersecting with the central axis m. This is also understood from the fact that the spherical aberration coefficient CS becomes large as r becomes large in a calculation equation of the spherical aberration coefficient CS as defined by Equation (1) as following:
                              [                      Equation            ⁢                                                  ⁢            1                    ]                ⁢                                                                                                C          S                =                                            F              4                        64                    ⁢                                    ∫              a              b                        ⁢                                                            V                  Z                                      1                    2                                                  ⁡                                  [                                                                                    {                                                                              3                            ⁢                                                                                          (                                                                                                      V                                    Z                                    ′                                                                                                        V                                    Z                                                                                                  )                                                            4                                                                                -                                                      4.5                            ⁢                                                                                                                            (                                                                                                            V                                      Z                                      ′                                                                                                              V                                      Z                                                                                                        )                                                                2                                                            ·                                                                                                (                                                                                                            V                                      Z                                      ′                                                                                                              V                                      Z                                                                                                        )                                                                ′                                                                                                              +                                                      5                            ⁢                                                                                          (                                                                                                      V                                    Z                                    ′                                                                                                        V                                    Z                                                                                                  )                                                            ′2                                                                                                      }                                            ⁢                                              r                        4                                                              +                                          4                      ⁢                                                                        V                          Z                          ′                                                                          V                          Z                                                                    ⁢                                                                        (                                                                                    V                              Z                              ′                                                                                      V                              Z                                                                                )                                                ′                                            ⁢                                                                        r                          3                                                ·                                                  r                          ′                                                                                                      ]                                            ⁢                                                          ⁢                              ⅆ                z                                                                        (        1        )            Herein, F is a focal length, z is a distance on the central axis, Vz is a potential on the central axis, Vz′ is a differentiation value of Vz with respect to z, a symbol ′ is differentiation with respect to z, r is a distance from the charged particle radiation to the central axis and is a function of z, and r′ is a differentiation value of r with respect to z.
Patent Literature 1: JP-A-Heisei 10-241616
Patent Literature 2: JP2000-340152A